Luminescent materials, termed phosphors, have general utility in a broad range of lighting and display applications. A phenomenon common to all such applications is excitation of the phosphors in accordance with any one of a number of techniques known in the art, causing the phosphors to emit light. Known excitation techniques include exposing the phosphors to emissions from an external energy source. The emissions can be in the form of electrons, ultra violet, x-rays, or gamma rays, to name a few. Hence, this excitation phenomenon is apparent in virtually all types of conventional, phosphor-containing, host lighting or display fixtures. Among such conventional fixtures are fluorescent tubes, cathode ray tubes, liquid crystal displays, gas discharge plasma displays, vacuum fluorescent displays, and field emission displays.
Cathode ray tubes are typical of luminescent displays employing electron emissions as the excitation means for the phosphors. Such displays have an anode panel coated with phosphors that are selectively excited by electrons directed toward the phosphors from an adjacent electron-supplying cathode. The excited phosphors emit light, thereby creating a desired image visible to the viewer on the screen of the display. Phosphors having utility for display applications typically comprise a host lattice impregnated with a quantity of a dopant that activates luminescent properties in the resulting composition. The phosphors are conventionally manufactured by selecting the host lattice and dopant from among well-known materials. The selected lattice and dopant are mixed together and milled to a relatively uniform particle size distribution. A typical average particle size for the mixture is on the order of about 10 microns because it is believed that such relatively large particle sizes contribute to the luminescent efficiency of the resulting phosphor product. A flux is also generally added to the mixture to facilitate subsequent heat treatment thereof. Fluxes having utility in the preparation of phosphors are characterized as materials having a relatively low melting point typically about 1000.degree. C. or less that promote infiltration of the dopant into the lattice structure when heated. Conventional fluxes include ammonium compounds, such as ammonium chloride, and compounds combining Group I A or II A elements and Group VI A or VII A elements, such as alkali metal halides and alkaline earth metal halides. Other agents facilitating heat treatment of the lattice and dopant mixture can also be combined with the mixture such as sulfur which serves as an antioxidant.
The composition comprising the host lattice, dopant, and flux, as well as any other selected treatment agents, is placed in a crucible formed from a refractory material, such as silica or alumina, and heated above the melting point of the flux to effectuate infiltration of the dopant into the host lattice. The presence of the flux, however, tends to induce excessive growth of the lattice particles during heat treatment. Consequently, the heat treated composition may be remilled following heat treatment to restore it to its original pretreatment particle size. Unfortunately, remilling the heat treated particles can negatively impact the luminescent efficiency of the resulting phosphor product by exposing surfaces of the phosphor product having relatively low dopant concentrations. In any case, a final step in the manufacture of phosphors is removal of the flux from the particles by means such as water or acid washing to obtain the desired phosphor product.
Although the above-described prior art process produces phosphors of adequate purity for many conventional display applications including cathode ray tubes, it has been found that present-day field emission display applications require phosphors of greater purity than those produced by such prior art processes. Specifically, it has been Found that residual quantities of flux unduly contaminate phosphor products manufactured in accordance with prior art processes even after washing the product. Contaminants retained by the phosphors from the flux, namely Group I A or II A cations, are often at times generally incompatible with silicon structures employed in the displays. More particularly, such contaminants can cause failure of field emission displays because the emitter tips that serve as the cathodes of a field emission display are extremely sensitive to contamination. The positively charged Group I A or II A cations are highly mobile in the evacuated environment of field emission displays. Group I A and II A cations readily migrate the relatively short distance from the anode plate to the cathodic emitter tips. An excessive accumulation of such cations on the emitter tips causes irreparable damage thereto. Refractory crucibles can likewise contribute Group I A or II A contaminants to the phosphor product due to their relative porosity that retains such contaminants and undesirably releases them into the phosphor product when heated.
As such, a need exists for a high-purity phosphor having specific utility to field emission display applications. Accordingly, it is an object of the present invention to provide a process for manufacturing a high-purity phosphor that satisfies the performance demands of field emission displays. More particularly, it is an object of the present invention to provide a process for manufacturing a high-purity phosphor that does not require a flux or any other treatment agent during heat treatment of the lattice and dopant. It is another object of the present invention to provide a process for manufacturing a high-purity phosphor, wherein the product is substantially free of contaminants from fluxes, other treatment agents, or process vessels which could diminish the operability of cathodic emitter tips employed with the phosphors in a field emission display. It is yet another object of the present invention to provide a process for manufacturing a high-purity phosphor having a relatively small particle size, yet having an acceptable luminescent efficiency. It is still another object of the present invention to provide a process for manufacturing a high-purity phosphor, wherein the lattice and dopant are heat treated at a relatively high temperature without substantially increasing the particle size thereof. It is a further object of the present invention to provide a process for manufacturing a high-purity phosphor, wherein the dopant is well distributed throughout the host lattice. These objects and others are accomplished in accordance with the invention described hereafter.